The most advanced development of the classic bulldozer is the front-end tractor shovel. This machine is also called a shovel dozer, a dozer shovel, a tractor loader, an end loader, a front loader, or just a loader. It is used for loading, roughgrading and limited hauling. A typical front-end loader includes: a support frame often mounted on the body of a tractor; a hydraulic system; a pair of push or lift arms, sometimes called a boom, hinged to the support frame; a tractor width bucket hinged to the front end of the lift arms; and a pair of dump arms hinged to both the lift arms and to the bucket.
The normal operation of front-end loader involves positioning the bucket in a "dig" or working position and then forcing the bucket into a pile of material by driving the tractor forward in low gear. When the resistance slows the tractor, the bucket is pivoted or "rolled back" on the lift arm while the lift arm is raised to force or break out the mass of material within the bucket from the remainder of the pile. Rolling back the bucket as it rises in the bank increases the cutting efficiency by aligning the leading edge with its upward movement. By retracting the bucket for a thinner slice, the bucket's own suction and crowding by the tractor tend to make the cut thicker. The proper balance among these forces varies with the loader being used, the slope of the soil bank, and the position and momentum of the bucket. The loader operator balances these various factors to efficiently operate the bucket and the loader to its optimum efficiency. Subsequently, the lift arms are elevated a sufficient distance to raise the bucket above the ground and then the tractor or machine is driven to some other location. Upon reaching the subsequent location, the bucket is pivoted to a "dumping position" where the contents are discharged after which the operation is repeated.
From the foregoing it should be appreciated that when the bucket is driven into the soil, the lift arms of the loader are fully extended to obtain the maximum thrust from the tractor. The greatest downward force is imposed at the bucket end of the loader arms when the bucket is pivoted during filling. Thus, a very large moment arm is imposed upon the tractor which must be overcome by the hydraulic actuators used to raise the lift arms.
The prior art is replete with loader designs incorporating lift arms operated by a hydraulic piston and cylinder with one end of the hydraulic actuator connected to the loader arm, at a point intermediate the ends of the loader arm, and connected at its other end to a fixed pivot point on the body of the tractor. Since the two pivoted ends of the hydraulic actuator are fixed relative to the tractor and the loader arm, the lifting moment produced by the hydraulic actuator generally varies with the angle of the loader arm relative to the horizon (i.e. angle of lift). Consequently, the hydraulic actuator is normally positioned relative to the fixed pivot point of the lift arm on the tractor such that the greatest lifting moment is produced when the lift arms in their lowered position. Thus, as the lift arms are raised to their highest position, the moment arm used to raise the lift arm decreases. One effect of this mounting arrangement is that the time it takes to raise the lift arms increases as the angle of lift increases.
What is needed is a lifting mechanism wherein the moment used to raise the lift arm is kept as high as possible throughout the lift cycle. Such a mechanism would improve the overall productivity of the loader and make the best use of the tractor's hydraulic system.